This invention relates to apparatus for coupling an electrical signal to transmission lines. Such apparatus is required either in data transmission systems or in voice communication systems. In data transmission systems, data communication equipment or datasets are widely employed with interconnection of two or more remote units of data terminal equipment such as computers, printers and video terminals. The dataset is usually located with and connected to the data terminal equipment and interconnected by paired wire lines such as telephone cable.
In order to avoid excessive cross talk in the telephone cable, balanced differential transmission is used to carry signals between the remote ends. In many installations, substantial ground potential differences may exist between the remote ends so that the dataset equipment must be able to detect the signal voltage which may be 100 times smaller than the ground potential difference. Such potential differences or common mode voltages originate from ground potential between the two locations and from induced electrical noise.
To handle the common mode signals, several techniques have been used up to date. Specifically, these are:
1. A direct coupled arrangement which relies solely on a differential to single ended convertor. Such convertors are conventionally only able to handle small common mode signals, especially with small, relatively economical receivers.
2. Transformer isolation has been used. However, this has the disadvantages that firstly AC signals must be transmitted, secondly the apparatus is expensive to manufacture and consumes considerable power and thirdly, it takes up valuable space which is expensive for the user and also for the manufacturer in providing the necessary casing, etc.
3. Optical isolators have also been used which are a family of components which in one package includes a light emitting device (usually a LED light emitting diode) which is directed upon a photo-sensitive device (usually a photo transistor, SCR or diode). The digital information is therefore sent over a light beam which provides complete electrical isolation. However, such apparatus has the disadvantages that it is expensive, distorts analog signals, consumes considerable power, requires an isolated supply and excessive cross talk can occur in cable facilities when optical isolation is used directly on the transmission line.